All About The Best Dynamo Hub Cache Batteries and Buffer Batteries

September 12th, 2017

If you’re keen on charging electronics, you’re probably already aware of the option to incorporate dynamo hub cache batteries (or buffer batteries) into your dynamo system. As long as your cache battery has some charge, it can provide continuous power to your devices when your speed drops too low for USB charging, such as when climbing a steep hill or when stopping at traffic lights.

Cache batteries will also help you stay off-the-grid for a significant amount of time provided you ride enough, you aren’t a heavy electronics user and you have a little bit of power on reserve.

The best dynamo hub cache batteries have a couple of key characteristics which we’ll cover in this resource.

USB chargers are best used in conjunction with cache batteries in order to supply a constant charge to your devices.

Pass-Through Batteries

Most batteries will either charge up via USB OR send their stored power to your device; rarely will they do both simultaneously. But, there is a special battery feature that makes both possible with some battery models.

Pass-through is the most important requirement of dynamo hub cache batteries. With this feature, the battery can actually send a charge to a device with no loss in efficiency. If your speed drops too low, the battery storage will pick up the difference and keep a constant charge to your device. When your device is full, the battery will be able to charge itself back up.

The reason many battery manufacturers don’t allow pass-through is that it can deteriorate batteries a bit quicker than otherwise. That said, you can expect a minimum of 500 total charge/discharge cycles (empty to full) with most batteries. And yes, two half charges equate to one full cycle.

The Goal Zero Flip 20 is a battery with low current pass-through charging suitable for dynamo hubs.

Battery Storage Losses

When power is stored in a battery, it experiences a storage loss of 15-20%. In order to minimise this loss, it is best to try to charge your electronics either straight from the USB charger or using the pass-through feature of a battery. It’s nice to have that power on tap, but when it means you lose 1-hour of power per 5-hour ride… it adds up!

Variable Current Dynamo Hub Cache Batteries

Some batteries are available with a variable current. What does this all mean? Is it important?

Well, current increases with your speed. At really low speeds, a dynamo hub will make 100mA current for example, but at high speeds, it can make greater than 1A current. With ‘current limiting’ batteries, you’re able to switch between set charging currents (eg. 100, 450 or 900mA).

The main advantage is that you can charge your battery with a constant current (eg. 100mA) at really low speeds (eg. 8km/h). However, the charging rate at 8km/h is approximately 5x slower than if you are traveling at 20km/h, so you’re not getting all that much power at those speeds anyway.

At higher speeds, the current limiter puts a ceiling on how many watts can be produced (eg. 3 watts @ 900mA). If you are likely to ride at speeds exceeding 25km/h+ then you’re better off without any current limit.

The Cinq5 Smart Power Pack II has a variable current for charging at low speeds.

How Long Will A Battery Take To Charge?

It’s not too hard to determine how fast a battery can charge, but we will need to make a few assumptions first.

A 3-watt dynamo hub will put out an average of around 6-volts at 500mA. Once the power has been converted from AC to DC the output will translate to roughly 5V and 500mA (the USB 2.0 charging standard), which for an hour of riding is 500mAh. Charging a 5000mAh battery will, therefore, equate to 10 hours of riding. However, there are also small losses in the charging circuitry, perhaps around 20% of the total generated power. Adding the 20% extra ride time (10 hours + (10 hours x 0.2)) and we’re looking at 12 hours of riding to fill the battery from scratch.

You can also use the above assumptions when determining how long it will take to fill a phone battery. My Apple iPhone 5S has a 1500mAh battery, so at 500mA per hour, it should take 3 hours. Adding in the circuitry losses and I’m looking at 3.6 hours of riding to fill it up. Other modern smartphones like the Samsung S8 will have 3000mAh batteries, resulting in 7 hour + charge times.

Having said all that, there are products which use special capacitors to achieve almost twice as much power as discussed. There is a downside though: additional dynamo hub resistance. Check out the Forumslader V5 and Dynamo Harvester Plus USB chargers which can achieve 5 watts (1A) power at 20km/h.

Battery Capacity

The battery size that you choose will depend on what devices you’re planning on charging, as well as how far you’re cycling per day. With most USB chargers you’ll be able to yield between 2500-3000mAh per 100km. To give you an idea of battery sizes in devices, a GoPro battery is 1220mAh, a Kindle battery is 1400mAh and smartphones tend to be between 1500-3000mAh.

Larger capacity batteries can be problematic to charge as they have more internal resistance in the battery cells. This requires a higher current (eg. 1A) to overcome the resistance. That’s no worries for a wall outlet, but can be hard to achieve with a dynamo hub at normal touring speeds. This can be a reason why some batteries won’t charge from a dynamo hub. So in summary, go smaller where you can, because the battery will likely be better able to match the inflow of current.

If you’re not planning on using battery storage, you can get away with as little as ~2000mAh in your cache battery. Remember that you’ll need more capacity if you’re likely to be dipping below 15km/h often.

Pre-Charging Batteries and Long-Term Storage

I fill up my cache battery from a wall outlet when I can, allowing me to use more power per day than I generate. By knowing roughly how big the batteries are inside my devices, I can ration my power (if required) so that my battery is almost depleted by the next wall outlet.

Many battery manufacturers recommend storing batteries with a 75% charge. This is because the chemicals inside are less active when left unused. To extend a battery’s life, it’s a good idea to discharge and recharge it at least once every 3-6 months too.

Cache Battery Recommendations

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Alee LOVES bikes and travel. So much so he once sold everything he owned to ride through 30 countries to learn a bit about himself and the world. Check out his book Bicycle Touring in One Hour to do the same: http://cyclingabout.com/bicycle-touring-book/

Great article Alee. I’ve had no end of trouble with my XT dynamo hub, Plug III and battery packs. No battery seems to charge, the L.E.D. just flashes which means it’s not working. I’m losing hope of (potentially!) living off-grid! It must be down to the battery packs I’ve tried as I have B&M dyno lights that work great. I’ve tried both a 2,000mAh and a 13,000mAh battery wth no luck. I’ve turned the lights off to ensure they aren’t sucking in power.

Do you have a wider range of cache battery suggestions suitable for cycling? The Goal Zero Flip models’ ‘flip’ charging port format doesn’t make them very suitable for use on a bicycle (e.g. particularly with Sinewave Reactor or Plug III). The Goal Zero Guide 10 Plus is a AA or AAA battery charger rather than a cache battery (although I presume it could be used as a cache battery, but it wouldn’t be a first choice for that purpose). The Powermonkey Explorer 2 looks needlessly bulky and heavy for cycling, although at least it should be weatherproof. The Cinq5 Smart Power Pack is a good compact cache battery (if pretty expensive) for cycling. We’ve been using a couple of these for 3 years, but my main criticism is that they are not very weatherproof, which seems to be a problem with the various generations of Cing5 Plug devices too. It is because I am becoming unhappy with the reliability of our Cinq5 SPPs that I’m now looking for alternatives. I’m loathed to buy another if Cinq5 can’t sort out the weatherproofing (and I carefully protect the USB ports from water). Of course, Anker and TechNet produce various cache batteries, and at a good price. However, it seems to be hit or miss whether any particular model allows ‘pass-through’ charging, and it’s often not stated either way in the product descriptions. Also, Anker and TechNet batteries aren’t designed for exposure to weather either. Any ideas about other suitable products would be appreciated.

The ones I’ve listed are tried and tested options in my bike community. I’ve heard whispers that the Patriot Fuel + 7800mAh (or 9000mAh) are a good batteries too for US $20-30. They’re available here: http://amzn.to/2haKRuq

Bahrd

An informative article. Thanks!
Is it possible to combine Alfine’s Di2 (or any other electronic shifting system) with a dynamo hub?

You can use the SM-BCR2 Di2 USB charger, but companies like kLite can also make a custom circuitry which removes the largish controller box with light indicators. My friend Jesse used kLite to build his dynamo Di2 charging setup: https://www.cyclingabout.com/video-trans-am-bike-race/

Bahrd

I have to admit, I used to think that the Alfine’s Di2 Motor Unit had its own small dynamo.

Clayton Wangbichler

Hey Alee. Article. Quick question about the Goal Zero Flip batteries. I have a Flip 20 and like the battery, but charging can be such a pain with it. The simple USB charger is nice when plugging into a wall outlet, but difficult (or impossible) when plugging into a Plug III, for example. How do you connect yours to charge while riding? Thank you.

Clayton Wangbichler

Do you find that as simple and reliable as the micro usb method that most batteries charge with? Micro USB are pretty easy to find internationally. Also micro usb charges lights and other devices, so it minimizes the amount of cords you have to carry while bikepacking. Thank you for your input. I’m trying to figure out if I should keep the Flip 20 or go with a different battery.

If you’re happy with the way your battery is performing, then just grab an extension cable. Shouldn’t add more than 30g to your setup.

Little Devils

You mention the Goal Zero Guide 10 Plus – any idea if this would work directly hooked up to a dynamo through the solar port? As it has a solar port, I wonder if it has some sort of regulator built into it? The specs of this port say Mini solar port (input): 6.5V, up to 1.1A – but how is that achieved if you just plug a solar panel directly into it? I can only think there must be some sort of regulator built into the unit, thus giving me hope I can directly connect it to a dynamo. What do you think?

The specs would indicate that it’s possible. I’m not too sure in practice though – let us know how you go!

Metten

I bought the goal zero flip 20 as a result of this article. Finally i thought to have a solution for my problem of failing charge at low speed and the flipping on and off from my Iphone. But at low speed my Iphone keeps flipping on and of because of charging/not charging. I use the plug with a Son dynamo on my Idworx bicycle. Although the goal zero is a pass through battery, it probably passes through the charging power directly to the device and not via the battery when connected. So it CANNOT be used as a buffer to prevent the problem. Nice powerbank, but no solution for the problem. Still looking for a solution 🙁

Interesting. They must have changed something with the battery because my Flip 20 works as I described. I assume your battery has a decent charge before you plug it in?

Metten

Yes, fully charged. It is strange indeed. The battery charges with the plug, the Iphone also, so all cables are ok. It just doesn’t charge the battery with priority, but the Iphone. When i take the phone off it charges the battery. When the speed drops the battery doesn’t take the charging over. Maybe it depends on the type of smartphone and the amperage it needs?